1. Field of the Invention
The present invention relates to High Intensity Discharge (HID) lamps, and more particularly, to an electronic ballast for use with an HID lamp.
2. Background and Related Information
Generally, there are two types of operation modes for HID lamps. In a first operation mode, generally referred to as a starting mode, a lamp arc tube (e.g., lamp) requires a certain high peak voltage, such as, for example, approximately 3 kV to approximately 5 kV, to ignite an arc between two electrodes in the lamp. In a second operation mode, generally referred to as a lighting mode, the lamp requires a certain RMS current that corresponds to an impedance between the two electrodes in the lamp in order to sustain (maintain) the arc, so that the lamp continues to emit light.
In a conventional electronic ballast, a full bridge circuit may be employed, to which a DC voltage, typically less than 500V, is applied. This conventional electronic ballast has a resonant igniter function for the lamp starting mode and a buck inverter function for the lighting mode.
The resonant igniter function provides the certain high peak voltage output, typically approximately 3 kV to approximately 5 kV peak, which is not constrained by the DC voltage of the full bridge circuit. During the starting mode, a resonance is generated between an internal inductance (inductor) and a capacitance (capacitor) of the electronic ballast. This resonance provides a high peak voltage across the inductor, which is applied to output terminals that are connected to electrodes of an un-ignited HID lamp. However, once the lamp is ignited by the certain high peak voltage, the current provided from the resonant circuit to the lamp load is usually insufficient to sustain the arc inside the lamp arc tube, as only a glow current flows through the lamp.
The buck inverter function (e.g., a circuit that performs DC to AC conversion) provides the necessary certain RMS current to sustain the arc, which is a smoothed DC current chopped at a low frequency (typically limited to a frequency of, for example, less than several hundred Hertz), that is provided to an output load (e.g., the ignited HID lamp). However, a maximum voltage OCV supplied by the circuit (electronic ballast) to the output load is limited by the applied DC voltage to the circuit. The maximum voltage OCV is generally not high enough to re-ignite the arc tube if the arc extinguishes.
Therefore, to operate an HID lamp, a conventional circuit (ballast) typically initially provides the igniter function to ignite the lamp in the starting mode, and then shifts its operation to the buck inverter function to sustain the arc in the lighting mode after it has been detected that the lamp is ignited.
Another operation mode exists between the starting mode and the lighting mode. This operation mode is known as a glow-to-arc transition mode, and occurs when the breakdown between the electrodes of the lamp has just occurred. The glow-to-arc transition mode typically lasts only a few hundred milliseconds to a few seconds. Some HID lamps require both a RMS current and a high peak voltage to heat up the electrodes in order to sustain the arc, or to re-ignite the arc in the event the arc goes out (is extinguished) due to the electrodes not being sufficiently warmed up to sustain the arc.
Unfortunately, the above-described conventional circuit does not provide appropriate output characteristics for the glow-to-arc transition mode. The igniter function of the conventional circuit provides a high peak voltage, but not enough sustaining current, while the buck inverter function provides an adequate RMS current, but not enough high peak voltage.
As a result, there is a shortcoming in the conventional circuit with respect to the igniter function and the buck inverter function. Specifically, the conventional circuit lacks the optimization of lamp glow-to-arc transition required for some HID lamps.